Aspects of the E Gi Eeri G Geology of Maputo City
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ASPECTS OF THE EGIEERIG GEOLOGY OF MAPUTO CITY, MOZAMBIQUE By EOQUE MEDES VICETE Submitted in fulfilment of the academic requirements for the Doctoral degree (PhD) in the School of Geological Sciences University of KwaZulu-Natal Durban MARCH 2011 ABSTRACT The geological formations of Maputo City, which are mainly unconsolidated materials with soil like properties, are described in terms of their engineering geological and geotechnical characteristics with relevance to their distribution patterns and spatial trends. Problematic conditions such as collapse potential characteristics, loose aeolian sand dune deposits and loose sand plains characterize many of the materials. The geological characteristics combined with anthropogenic interference such as intensive urbanization with inappropriate land use, construction in sensitive areas like steep sandy slopes has led to many problems including slope stability. Foundation problems with building settlement and gully erosion also occur. The aim of this research was to study the engineering geological characteristics and the geotechnical properties of the geological formations of Maputo City and various related problems. Special relevance has been given to the understanding of three specific problems: building damage, gully erosion and slope instability. The geological formations are predominantly sandy (coarse to very fine sand) with very low clay content, are non-plastic and are classified as from the group SP-SM which are poorly- graded sand with silt. The majority of the materials are loose and normally consolidated with a high level of residual strength. Assessment of collapse settlement through double consolidation technique indicated soil compressibility and significant sensibility to collapse upon wetting. Truly collapsible soils that show full collapse of the soil structure were identified in 33% of the tested materials where the highest collapse behaviour reached values above 5%, predicted to cause moderate trouble in foundation design. Some of the bonded materials are bonded (evident in 67% of samples tested). Bonding was confirmed by comparing the compressibility of the undisturbed and remoulded samples. The remoulded samples showed a significantly higher compression than that of the bonded materials as part of the stress applied is carried by the bonds themselves, as the bonded material is stiffer than the same without bonds. The curves of the remoulded samples were used to establish the limit between the stable and meta-stable states of the material. A qualitative evaluation of the erosion susceptibility was investigated by physical tests such as the crumb test, shear strength and chemical indicators while a quantitative evaluation of the erodibility characteristics was obtained using a flume test. Some correlations were found between the results of various methods. Almost all samples that were found to be dispersive with ESP were also dispersive with TDS vs. %Na and SAR. Results of the flume erodibility ii test have very little correlation with the chemical properties related to dispersion revealing that the erosion susceptibility and gullying in Maputo City have more relation to the physical processes than to the dispersion related chemical properties of the soils. The positive identification of dispersive and erodible soils can only be carried out using a combination of various techniques. Therefore, a new rating system for erosion susceptibility of sandy soils combining the physical and chemical factors of dispersion is proposed including the flume test, crumb test, TDS/%Na, SAR and ESP. The proposed rating system was applied to the tested soils of Maputo City. Fifteen samples (83% of the rated samples) were classified with intermediate susceptibility to erosion while 3 samples (17%) were classified as having a low susceptibility to erosion. The highest rating scores were obtained by the same samples that showed dispersive behaviour with SAR, ESP and TDS/%Na. This group of samples was of intermediate erodibility in the flume test. The slope instability mechanisms observed in Maputo City are predominantly rotational failures with a mass of soil sliding along a curved surface of rupture followed by sand flow at the toe as failure occurs in the presence of excess water. Four groups of factors account for the slope instability problems in Maputo City: geomorphological causes, physical and meteorological causes, geological and geotechnical properties of soils, and anthropogenic causes. The mechanism of failure is mostly due to the loss of matric suction of soils in the presence of rainwater and possibly from destruction of bonding agents. Factors of safety values indicate that the slopes are generally unstable with the control being the slope angle. The slopes in the Polana-Caniço and Ferroviário Quarters show high factor of safety values but is the area most affected by slope instability. Slope failure in these areas is intrinsically caused by anthropogenic factors related to inappropriate land use planning. The gully sidewalls are unstable as the slope created is very steep. The slope at Friedrich Engels Avenue causes most concern due not only to the slope height and angle but also to the size and number of buildings constructed at the crest, mainly high rise buildings along the Julius Nyerere Avenue, the integrity of which could be threatened by a landslide event (this slope has recently been affected by active landslides). iii PREFACE The research work described in this thesis was carried out in the School of Geological Sciences, University of KwaZulu-Natal from April 2004 to December 2009 under the supervision, first of Prof Deneys Schreiner from the School of Civil Engineering, Surveying and Construction and Prof Colin Jermy and in the last year under the supervision Dr Nick Richards. This thesis represents original work by the author and has not otherwise been submitted in any form for any degree or diploma to any tertiary institution. Where use has been made of the work of others it is duly acknowledged in the text. Enoque Mendes Vicente 11 March 2011 iv FACULTY OF SCIECE AD AGRICULTURE DECLARATIO 1 – PLAGIARISM I, EOQUE MEDES VICETE , declare that 1. The research reported in this thesis, except where otherwise indicated, is my original research. 2. This thesis has not been submitted for any degree or examination at any other university. 3. This thesis does not contain other persons’ data, pictures, graphs or other information, unless specifically acknowledged as being sourced from other persons. 4. This thesis does not contain other persons' writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted, then: a. Their words have been re-written but the general information attributed to them has been referenced b. Where their exact words have been used, then their writing has been placed in italics and inside quotation marks, and referenced. 5. This thesis does not contain text, graphics or tables copied and pasted from the Internet, unless specifically acknowledged, and the source being detailed in the thesis and in the References sections. Signed ………………………………………………………………………… v FACULTY OF SCIECE AD AGRICULTURE DECLARATIO 2 – PUBLICATIOS DETAILS OF CONTRIBUTION TO PUBLICATIONS that form part and/or include research presented in this thesis (include publications in preparation, submitted, in press and published and give details of the contributions of each author to the experimental work and writing of each publication) Publication 1: Vicente, E. M. ; Jermy, C. A. & Schreiner, H. D. (2006). Urban Geology of Maputo, Mozambique. Proceedings of the International Association of Engineering Geologists Congress. 8. Nottingham, 12 pp. The candidate was the first author of this conference paper. This paper was a compilation of the research topic sent to the university during the application process for the PhD studies. The co- authors are the first supervisors of this research project. All the data, the field work in Maputo and laboratory testing at University of KwaZulu-Natal was done by the candidate as a part of this research. Publication 2: Vicente, E. M. ; Amurane, D. P. & Xerinda, L. (2006). Assessment of slope stability in Maputo City, Mozambique. Proceedings of the International Association of Engineering Geologists Congress. 8. Nottingham, 9 pp. The candidate was the first author of this conference paper. All the data, the field work in Maputo and laboratory testing at the University of KwaZulu-Natal was done by the candidate as a part of this research. The co-authors were only involved in the field work. Signed: vi LIST OF COTETS Abstract ii Preface iv Declaration 1 – Plagiarism v Declaration 2 – Publications vi List of Figures xi List of Tables xiv Acknowledgements xvi CHAPTER OE 1 1 – ITRODUCTIO 1 1.1 – Background and Research Topic 1 1.2 – Research Objectives 4 1.3 – Hypotheses to Be Tested 4 1.4 – Research Definition 6 1.5 – Future Research 11 CHAPTER TWO 13 2 – LITERATURE REVIEW 13 2.1 – Gully Erosion 13 2.2 – Buildings Settlement and Collapsible Soils 15 2.3 – Slope Instability 17 CHAPTER THREE 20 3 – RESEARCH METHODOLOGY 20 3.1 – Sampling Methodology 20 3.2 – Laboratory Testing Process 22 3.2.1 – Testing Physical Properties For Soil Classification 23 3.2.1.1 – Particle Size Distribution 23 3.2.1.2 – Consistency Limits (Liquid And Plastic Limits) 24 3.2.1.3 – Moisture Content 25 3.2.1.4 – Particle Density And Specific Gravity 26 3.2.1.5 – Organic Matter 26 3.2.2 – Determination Of Shear